Heretofore, the following procedure has been known. Specifically, natural gas and water are reacted with each other at a temperature higher than the freezing point at a pressure higher than the atmospheric pressure to form a natural gas hydrate without freezing water. The natural gas hydrate thus formed is physically dewatered. Then, the water content of the natural gas hydrate is reduced by reacting natural gas with the residual water content contained in the natural gas hydrate during the physical dewatering step or after the dewatering to generate a natural gas hydrate. The resultant is cooled to a temperature lower than the freezing point, followed by depressurizing (see, for example, Patent Document 1).
However, in a case where the gas phase at the physical dewatering means, a transferring section, or the like has a natural gas composition in this production system, an additional gas hydrate may be generated from heavy components (ethane, propane, butane, and the like) contained in the composition. This may result in operation problems such as transferring failure in some cases.
To inhibit the occurrence of such operation problems, the gas phases in facilities downstream of the generation step have to be in an equilibrium state with hydrate and water, in other words, the gas phases have to have the same gas composition as that in the generation tank. As an invention analogous to this, for example, Patent Document 2 is known.
However, this invention requires a large-scale auxiliary facility for adjustment of a mixed gas supplied to a generation tank by dilution with a main component of the mixed gas, that is, requires a large-scale auxiliary facility including the control system. Further, the adjustment to the equilibrium composition is difficult under generation conditions, and there are still problems such as that a gas hydrate may be generated in the downstream facilities.